1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
There are some power conversion circuits which accomplish higher efficiencies by implementing a mechanism that accomplishes switching at zero voltage. Power loss in a switch is the product of the voltage applied across the switch and the current flowing through the switch. In a switching power converter, when the switch is in the on state, the voltage across the switch is zero, so the power loss is zero. When the switch is in the off state, the power loss is zero, because the current through the switch is zero. During the transition from on to off, and vice versa, power losses can occur, if there is no mechanism to switch at zero voltage or zero current. During the switching transitions, energy losses will occur if there is simultaneously (1) non-zero voltage applied across the switch and (2) non-zero current flowing through the switch. The power losses associated with the switching transitions will be the product of the energy lost per transition and the switching frequency. The power losses that occur because of these transitions are referred to as switching losses by those people who are skilled in the art of switching power converter design. In zero voltage switching converters the zero voltage turn off transition is accomplished by turning off a switch in parallel with a capacitor and a diode when the capacitor's voltage is zero. The capacitor maintains the applied voltage at zero across the switch as the current through the switch falls to zero. In the zero voltage transition the current in the switch is transferred to the parallel capacitor as the switch turns off.
The zero voltage turn on transition is accomplished by discharging the parallel capacitor using the energy stored in a magnetic circuit element, such as an inductor, and turning on the switch after the parallel diode has begun to conduct. During the turn on transition the voltage across the switch is held at zero clamped by the parallel diode. The various zero voltage switching (ZVS) techniques differ in the control and modulation schemes used to accomplish regulation and in the energy storage mechanisms used to accomplish the zero voltage turn on transition.
One of the ZVS techniques uses a resonant circuit which is frequency modulated over a broad frequency range. An example is shown in FIG. 1. These techniques have been refined by a multi-resonant technique in which more resonant circuit elements and a complex control circuit are required, but the converter can operate at a fixed frequency.
Several techniques have been introduced which accomplish zero voltage switching, inherently, at constant switching frequency. One of these techniques requires a full bridge switching arrangement with four primary switches in which the regulation is accomplished by phase modulation or by alternating pulse width modulation in the two switching legs. This technique is illustrated in FIG. 2. This technique has become a standard technique for high power conversion at high frequency. One of the potential problems with this technique is staircase saturation of the transformer core resulting from relatively small DC imbalances in the transformer's primary winding, which can lead to catastrophic failure. One common solution to the staircase saturation problem is to place a capacitor in series with the primary winding of the transformer to block any DC current. The series capacitor incurs losses during high power operation and requires the user to use voltage mode control rather than the preferred current mode control, which unbalances the capacitor voltage resulting in high switch stress that can lead to catastrophic failure. Another problem is high conduction losses that results from the peak recirculation current during the reset time of the output choke.
The FIG. 3 circuit is an example of prior art that overcomes the staircase saturation problem associated with the FIG. 2 circuit. The two transformers are coupled inductors and energy storage devices that accommodate large DC currents, so that staircase saturation is not a problem. The FIG. 3 circuit can accomplish zero voltage switching under the right circumstances. The transitions are driven by the stored energy in the parallel inductor. The amount of energy stored in the parallel inductor must be large enough to charge/discharge the parasitic capacitors associated with the switches that are in transition. The current in the parallel inductor would have to be increased sufficiently to both provide current to drive the transition and current to drive the primary windings of the transformers. This is a particular problem at high line voltage where the energies required to drive the switching transition are greatest. For illustration, referring to FIG. 3, consider the condition in which switches S1 and S4 are on and switches S2 and S3 are off. Increasing current will flow from left to right through both the parallel inductor, L1, and the primary windings of the transformers. During this time current flows in the secondary winding of T1 through D1 and to the output capacitor and load resistor. Stored energy builds up in the core of transformer T2, but no current flows in the secondary winding of T2, since its secondary winding voltage reverse biases D2. After a time, switch S1 is turned off and the stored energies in L1 and T2 drive the transition which can easily be made to be zero voltage. During the state which follows the connection point between T1 and T2 drops below ground potential and the T1 primary voltage becomes equal to the sum of the voltages across T2 and the switches, S2 and S4. During the time that S2 and S4 are on, the current in L1 remains relatively unchanged, dropping slightly, but the current in the primary windings drops towards zero as the current in the secondary windings shifts from T1 to T2. The critical switching transition occurs when switch S4 turns off. During the switching transition that follows L1 must provide all of the energy to drive the transition, charging the output capacitance of S4, discharging the output capacitance of S3 and providing charge to the other parasitic capacitances in the windings of each of the magnetic circuit elements and the D1 diode, which becomes reverse biased during the transition. As secondary current shifts from D1 to D2 the current in the primary circuit reverses sign and flows from right to left. When the transition is complete the current in the primary winding will equal the magnetizing current in the primary winding of the T1 transformer. In this discussion, and all the discussions that follow, the magnetizing current will mean the current in a coupled inductor winding that is substantially proportional to the field of magnetic induction that exists in the core of the coupled inductor. The magnetizing current in a coupled inductor may be referred to any winding of that coupled inductor in such a manner that the total stored magnetic energy in the core of the coupled inductor is equal to one half times the inductance of the winding, to which the magnetizing current is referred, times the square of the magnetizing current. With this definition of magnetizing current the magnetizing current will have both AC and DC components, in general. As the switching transition progresses the current required by the primary circuit from L1 increases as the current provided by L1 decreases. The rate of increase of current in the primary windings from right to left during the S4 turn off transition depends on the line voltage and the resistance in the active section of the circuit. As the current in the primary windings increases from right to left much of the current and energy provided by the L1 inductor is diverted to driving the load. In order for the current in the inductor L1 to drive the load during the transition and also drive the transition the current provided by the inductor L1 must be larger than the peak primary current and the energy stored must be sufficient that the current provided by L1 is relatively invariant for the duration of the transition. As a result of the large current in L1 the conduction losses in the four switches are substantially increased by the presence of L1 and because of the substantial stored energy requirement the inductor L1 adds additional cost, weight and volume to the converter.